1. Field of the Invention
The present invention relates to a vortex flowmeter and, more particularly, to a vortex flowmeter for use in an internal combustion engine.
2. Description of the Related Art
In general, when a vortex flowmeter is employed in an internal combustion engine, it is always provided at the downstream side of an air cleaner provided to remove dust from air sucked into the engine, as shown, for example, in Japanese Patent Publication No. 62-26686 and Japanese Patent Public Disclosure No. 58-21517.
Incidentally, if the flow of a fluid which is to be measured is not stable, measuring accuracy thereof will be lowered and in some cases measurement may not be possible at all. Since a vortex flowmeter employed in an internal combustion engine is provided at the downstream side of an air cleaner, as described above, it is in many cases impossible to ensure sufficient space for a fluid to flow with the required level of stability and consequently drift and turbulent flows increase considerably when the flow rate is high. If a known straightening mechanism is employed in such an arrangement, generation of vortices may be obstructed.
FIG. 1 is a sectional view of a conventional vortex flowmeter 1 which is provided at the downstream side of an air cleaner of an engine. The vortex flowmeter 1 comprises a duct 11 having a quadrilateral cross-sectional configuration for passing a fluid which is to be measured, a first vortex generating column 12 provided inside the duct 11 to generate a Karman vortex, a second vortex generating column 13 provided inside the duct 11 at the downstream side of the first vortex generating column 12 to generate a Karman vortex, the second vortex generating column 13 having a vortex detecting pressure introducing port, a honeycomb straightening 14 provided at the upstream end of the duct 11, and a control circuit 15 provided outside the duct 11. An air cleaner 2 comprises an upstream cover 21 having a fluid inlet, a downstream cover 22 having a fluid outlet which is connected to the duct 11, and a dust removing element 23 provided between the upstream cover 21 and the downstream cover 22. An intake pipe 3 is connected to the downstream end of the duct 11 to lead a fluid to the engine through a throttle valve (not shown).
In the above-described arrangement, a fluid which is to be measured, that is, air, flows into the upstream cover 21 of the air cleaner 2, as shown by the streamline F.sub.IN, and then flows inside the downstream cover 22, as shown by the streamlines F.sub.1 to F.sub.4, to reach the inlet of the vortex flowmeter 1. Since the fluid tends to flow through a region where the resistance is relatively low, in general the air flow along the streamline F.sub.2 has the highest flow velocity, those along the streamlines F.sub.1 and F.sub.3 follow it, and the air flow along the streamline F.sub.4 has the lowest flow velocity. The velocity of the air flow along the streamline F.sub.4 is extremely unstable. The fluid reaching the inlet of the vortex flowmeter 1 flows into the intake pipe 3 along the streamline F.sub.OUT.
The following is a description of the flow velocity distribution in the vortex flowmeter 1 of the fluid streams flowing along the streamlines F.sub.1 to F.sub.4 in the air cleaner 2. FIG. 2 is an enlarged sectional view showing the outlet side of the air cleaner 2 and the upstream side of the vortex flowmeter 1. Reference numeral 11a in the figure denotes a bell mouth portion which is provided along the entire circumference of the inlet of the duct 11, the bell mouth portion 11a being disposed at the downstream side of the honeycomb straightening device 14. Accordingly, after reaching the honeycomb straightening device 14, the fluid streams flowing along the streamlines F.sub.1 to F.sub.4, which would otherwise flow in the respective directions shown by the chain lines, are straightened by the honeycomb straightening device 14 so as to flow in the respective directions shown by the solid lines. Then, the fluid streams along the streamlines F.sub.1 and F.sub.3 are accelerated in the bell mouth portion 11a so that the flow velocities of these fluid streams approach that of the fluid stream along the streamline F.sub.2 that has the highest flow velocity. Accordingly, if it is assumed that there is no fluid stream along the streamline F.sub.4, the flow velocity distribution inside the duct 11 immediately in front of the first vortex generating column 12 is relatively uniform, as shown by the solid line V.sub.L. In actuality, however, there is a fluid stream flowing along the streamline F.sub.4, and the fluid stream along the streamline F.sub.3 is therefore forced to shift downwardly by the fluid stream along the streamline F.sub.4, resulting in a reduction in the flow velocity of this fluid stream in the vicinity of the bell mouth portion 11a. Thus, the flow velocity distribution is distorted, as shown by the chain line V.sub.L '. A vortex that is generated when the flow velocity distribution is distorted as described above varies in intensity and sometimes disappears. Such a vortex condition is shown in FIG. 3. V.sub.C shown in FIG. 3(a) denotes the center of a vortex column generated in the duct 11, that is, the position of the vortex line. V.sub.O shown in FIG. 3(b) denotes the intensity of the vortex, that is, the vortex pressure. It is assumed that six vortices V.sub.1 to V.sub.6 are successively generated while the time T elapses from the right to the left as viewed in the figure. In the vortex V.sub.1, the distortion of the vortex line is not yet large. However, as the vortices V.sub.2 to V.sub.4 are succesively generated, the degree of distortion increases, and the vortex line finally breaks in the vortex V.sub.5. In the meantime, the vortex intensity V.sub.O gradually decreases and reaches zero at the time of generation of the vortex V.sub.5 in which the vortex line breaks. More specifically, there is practically no vortex V.sub.5. After the vortex line has broken, a vortex V.sub.6 having a relatively low vortex intensity V.sub.O is generated, and the vortex intensity V.sub.O gradually increases thereafter. However, the vortex disappears again after the vortices V.sub.1 to V.sub.4 have been successively generated.
Thus, the conventional vortex flowmeter suffers from the following problems. The vortex disappears periodically, and the accuracy of the measurement is substantially lowered in the case of a fluid which is likely to cause many drift and turbulent flows, with the end result that the vortex flowmeter fails to serve its purpose.